Control systems, such as for example, those utilizing programmable logic controllers (PLCs), can control a broad number and type of devices, including alarms, indicators, actuators, motors, relays, and even input devices, such as sensors. Often, a central processing unit (CPU) provides a control signal to an output module, which then provides a device control signal that is of a voltage appropriate to its particular device.
When a failure or shutdown of a typical control system, PLC, and/or CPU occurs, the output modules are deactivated, and their device control signals forced to zero voltage. Yet, a zero volt or lack of control signal is not necessarily a safe state for all controlled devices. That is, forcing a device control signal to zero might not leave the controlled device in a safe operating state. For example, to keep a certain space safely ventilated, a fan might need to keep operating, even when its control system fails or shuts down. Yet, if the control signal to the fan disappears, drops to a zero voltage, etc., the fan will also shut down.